This large application has a memory pool library which uses a treap internally to store nodes of memory. The treap is implemented using cpp macros, and the complete file trp.h can be found here. I get the following compiler warning when I attempt to compile the application: warning: this decimal constant is unsigned only in ISO C90 By deleting portions of the macro code and using trial-and-error, I finally found the culprit: #define trp_prio_get(a_type, a_field, a_node) \ (2654435761*(uint32_t)(uintptr_t)(a_node)) I'm not sure what that strange number is doing there, but I assume it's there for a good reason, so I just want to leave it alone. I do want to fix the warning though- any idea why the compiler's saying that it's unsigned only in ISO C90? EDIT: I'm using gcc-4.1